A conventional valve timing control device controls a relative phase between a crank shaft and a cam shaft, which is hereinafter referred to as an “engine phase,” to determine a valve timing by applying torque generated by an electric motor or an electromagnetic actuator such as an electromagnetic brake device to a phase control mechanism. JP2005-146993A describes a valve timing control device that ensures start performance of an internal combustion engine by retaining the engine phase of the internal combustion engine, which is in a stop state, in a start phase allowing start of the internal combustion engine.
The control device is provided with an electromagnetic brake device that operates during the stop state of the internal combustion engine to apply brake torque from a brake shaft to the phase control mechanism. As a result, in the stop state of the internal combustion, the brake torque of the electromagnetic brake device acting on the brake shaft and the spring torque of the phase control mechanism are made to be balanced, thereby retaining the engine phase in the start phase.
The above described electromagnetic brake device that operates at the stop state of the internal combustion engine is provided in addition to an electromagnetic brake device controlling the engine phase by applying the brake torque from the brake shaft to the phase control mechanism in an operating state of the internal combustion engine. Therefore, such a device leads to increased complexity and size of the device structure. In the operating state of the internal combustion engine, cam torque alternating between a positive and a negative direction in response to rotation of the cam shaft is transmitted through the phase control mechanism to the brake shaft. The cam torque acts on the brake shaft as positive torque or negative torque while the internal combustion engine is in the stop state. The torque balance in the brake shaft is disrupted depending upon the positive or the negative direction of the cam torque or the magnitude of the cam torque, possibly creating a large shift of the engine phase from the start phase.
The present application is associated with using an electric motor as an electromagnetic actuator by generating a magnetic retaining torque such as cogging torque in a motor shaft even when power is removed due to stopping of the internal combustion engine. According to various exemplary embodiments, in an operating state of the internal combustion engine, the engine phase is controlled by applying motor torque to a phase control mechanism. The motor torque is generated in the motor shaft by supplying power to the electric motor. In the stop state of the internal combustion engine, the engine torque is retained to the start phase by balancing the magnetic retaining torque generated in the electric motor with the cam torque. By controlling the engine phase in the operating state of the internal combustion engine and the retaining of a desired engine phase when the engine is in the stop state with the electric motor, it is possible to achieve simplification and downsizing of the device structure.
According to various studies conducted in connection with the present application, since the magnetic retaining torque alternates between a positive and a negative torque direction in response to the rotation of the motor shaft, it becomes difficult to balance the torques when a direction of the cam torque is in agreement with a direction of the magnetic retaining torque whether in a positive or negative direction, at a point where the motor torque is eliminated by the removing the power. Further, in a state where the magnetic retaining torque and the cam torque can not be balanced, it has been discovered that the motor shaft is made to rotate and therefore, when the magnetic retaining torque is averaged by alternation thereof between the positive and negative direction, damping of the rotation can not be obtained.